The present invention relates to an improved process for the production of unsaturated ethers, in particular isopropenyl methyl ether, by pyrolysis of a ketal-containing or acetal-containing mixture, in particular dimethoxypropane, in the presence of an organic carboxylic acid.
Various unsaturated ethers are important starting compounds for the production of pharmaceutical products, fragrances and perfumes. Isopropenyl methyl ether (IPM) is such an ether and may be used in particular for synthesizing vitamins, including inter alia the synthesis of vitamin E and vitamin A, and for producing various carotenoids such as astaxanthin and related compounds. IPM may furthermore be used for the synthesis of fragrances and perfumes. In this connection processes are known for the C-3 extension of allyl alcohols or propargyl alcohols, in which at least two equivalents of unsaturated ether are used per mole of substrate. In this reaction, which is also termed the Saucy-Marbet reaction after its discoverers, one equivalent of the ether is used for the C-3 extension of the substrate and one equivalent is used to trap the alcohol produced in situ, with formation of the ketal.
The production of acetals or ketals from the corresponding alcohols and carbonyl compounds, and the production of the unsaturated ethers from the resultant acetals and ketals, is described in the literature. The prior art is discussed here separately for ketalization and ketal pyrolysis for the production of the unsaturated ethers. It is generally known that dimethoxypropane (DMP) can be prepared from acetone and methanol by reaction in the presence of an acid catalyst.
Lorette et al., J. Org. Chem., 1959, p. 1731, describes the dependence of the educt stoichiometries and the temperatures on the kinetics of this reaction, in particular the fact that the equilibrium of this reaction is displaced predominantly towards the educts and accordingly it is not possible under moderate conditions to achieve a complete educt conversion. It is found that in order to achieve good educt conversions, low temperatures, i.e. down to xe2x88x9230xc2x0 C., have to be established, with an acetone/methanol ratio of 1:2 to 1:4. The ketalization of Lorette et al. is performed in the presence of acidic ion exchangers, the reaction being carried out as a fixed bed catalysis reaction. The working up of the product solution that is produced, the water content of which is between 3 and 4 wt. %, is complicated due to the formation of azeotropes between DMP and methanol on the one hand and acetone and methanol on the other hand, and the yields of isolated DMP are correspondingly low due to the losses.
U.S. Pat. No. 2,827,495 of Bond et al. is concerned with the working up of the aqueous, methanolic product mixture by extraction with aqueous alkali, in particular sodium hydroxide in a concentration between 13 and 16 wt. %. By this process, which is carried out industrially as a countercurrent extraction process, a methanol-free, almost pure DMP (97%) can be obtained as organic product of the extraction in an outstanding extraction yield ( greater than 99%, see Example IV of the cited patent). Nevertheless, the DMP extracted in this way still contains about 1.5 wt. % of water, which in molar terms corresponds to a ratio of 8:92. This mixture cannot however be used to produce IPM in large yields since the water that is present reacts quantitatively with DMP during the pyrolysis, in the presence of an acid catalyst, to form acetone and methanol. The further product purification and the use of the resultant DMP for the production of IPM is not described.
In U.S. Pat. No. 1,850,836 of Guinot et al. two basic possibilities for the production and isolation of acetals are already described, namely the reaction of an aldehyde with an alcohol in the presence of a catalytic amount of a mineral acid, in particular gaseous HCl. After the reaction equilibrium has been established the reaction mixture is neutralized with an amount of base at least equivalent to the acid (in order to suppress the reverse reaction during the working up) and is then worked up by adding an aliphatic auxiliary solvent that is water-insoluble and forms a minimum temperature azeotrope with the alcohol that is used. In this way the nonpolar acetal can be freed by means of the non-water-soluble aliphatic solvent from water and largely from the alcohol, and the alcohol can then be extracted as an azeotrope with the aliphatic compound. It is obvious that considerable amounts of aliphatic compounds are required for the complete removal of the alcohol, following which an aqueous extraction is necessary to separate the methanol from the aliphatic solvent. Overall the process is complicated and is not particularly suitable for industrial application.
In U.S. Pat. No. 2,837,575 of Waters et al. a ketalization with gaseous HCl is described. In order to increase the acetone conversion, up to 8 wt. % of HCl is used, which then has to be neutralized with sodium hydroxide, and a not inconsiderable amount of salt is formed. The subsequent working up is performed by two complicated extractions with sodium hydroxide of different concentrations followed by an additional extraction with a readily volatile aliphatic hydrocarbon. It is clear from the large number of necessary separation operations that the process is not suitable for an economic industrial application.
After the implementation of the ketalization per se had been technically solved by the publication by Lorette et al., J. Org. Chem., 1959, p. 1731, using a stable, acidic ion exchanger, the subsequent relevant patent publications were accordingly only concerned with the recovery of the complex product mixture, which due to the presence of water tends to undergo a reverse reaction and thus complicates the recovery still further.
According to the process described in DE-OS 26 36 278, Zinke-Allmang et al., BASF, the reaction of alcohol and carbonyl compound is carried out in the presence of gaseous HCl and at least equivalent amounts of calcium sulfate as water-binding agent. The same authors concede however in a later publication, DE 29 29 827, Zinke-Allmang, BASF, that this does not represent an optimal solution to the problem, since considerable amounts of the water-binding agent are used and have to be recovered. DE-OS 29 29 827 describes the reaction in an excess of acetone with an acetone/methanol ratio of 3.6 to 4.4, and recovery in a distillation column with 40-60 trays. The azeotrope of acetone and methanol is recycled at the head of the column and a mixture of DMP and water is extracted in a side stream, following which DMP can be obtained after phase separation. In this procedure however an only approximately 4 wt. % DMP mixture with a water content of ca. 0.5 wt. % is produced due to the establishment of the equilibrium. It is clear that almost 95 wt. % of the unreacted product solution has to be distilled off in order to isolate the product, which makes the process extremely energy-intensive, and the spatial requirements of the necessary distillation column can be extremely large, having regard to the recycling required for a clean separation.
U.S. Pat. No. 4,775,447 describes a process for the production of DMP, in which an acidic heterogeneous ion exchanger is likewise used as catalyst and the ratio of acetone to methanol is adjusted to between 1:1 and 1:3. The recovery according to this procedure is however extremely complicated and involves a first distillative removal of an acetone-rich azeotrope of acetone and methanol. A corresponding amount of acetone must be added to the remaining mixture of methanol and DMP so that, in a second distillation, an azeotrope of acetone and methanol of the composition (ca. 86 vol. % acetone and 14 vol. % methanol) is removed. The patent does not discuss the acetone-methanol separation. Also, the separation from the DMP of the water formed in the reaction is not described.
The distillative separation as well as the complex azeotropes in the water-DMP-acetone-methanol system are described in Brunner and Scholz, Chem. Ing.-Tech. 52 (1980), No. 2, pp. 164-166, as well as in Beregovikh et al., Khim. Farm. Zhl. 17 (1983), pp. 454-459.
Brunner and Scholz question the earlier results of Lorette et al. concerning the existence of a ternary azeotrope of acetone, methanol and DMP. Brunner and Scholz come to the conclusion that an acetone-rich azeotrope (composition as described above) of acetone and methanol having a boiling point of 55.4xc2x0 C., as well as a further azeotrope of methanol and DMP having a boiling point of 61.0xc2x0 C. and a composition of 72.5 mole % of methanol and 27.5 mole % of DMP exist.
The production of unsaturated ethers from the corresponding acetals or ketals is also described in the literature. U.S. Pat. No. 2,667,517 describes the pyrolysis in the presence of an acid catalyst from the group comprising sulfonic acids, in a hydrocarbon or a chlorinated hydrocarbon as solvent and/or diluent. Problems arise however in this procedure due to the high boiling point compounds that are formed in the reaction, which contaminate the catalyst solution and necessitate a high discharge rate. EP-A 0 197 283 describes the use of a mineral oil that is combusted after use, which however involves a not inconsiderable specific consumption of the catalyst.
It is also possible to operate in the absence of a solvent if, as proposed in DE-A 40 39 950, the pyrolysis is conducted at elevated temperatures up to 200xc2x0 C. in the presence of a catalyst system comprising an acid on the one hand and an amine on the other hand. The process is not generally applicable and has the disadvantage that here too the catalyst phase is contaminated by the non-selective reaction and thermal lability of the substances that are employed, which again necessitate a discharge.
EP-A 0 703 211 suggests a solution to these problems. By using high boiling point branched organic acids, the reaction can be carried out at temperatures between 100xc2x0 C. and 250xc2x0 C., whereby only a few high boiling point byproducts accumulate in the catalyst solution due to the highly selective nature of the reaction, which have only a slight influence on the catalytic activity of the system.
Sterically demanding substituted neoacids with 9 or more hydrocarbon atoms are used as preferred acids. The working up of the complex product mixtures of IPM, methanol, DMP and acetone is not discussed. It is also not clear what effect the water entrained with the DMP that is fed in has on the catalytic activity.
A common feature of all these processes is that the isolation of the product mixture is greatly complicated on account of the azeotrope with methanol that is formed during the distillation, and in addition there is also a back-reaction of DMP with water unless appropriate measures are adopted (distillation under basic conditions, column provided with Ca(OH)2), which however also represents a considerable expense. In the processes listed in the prior art a high product purity is always particularly sought after in the isolation of the ketal, since on account of the water that is fed in together with DMP into the pyrolysis (IPM) stage there is a back-reaction of the ketal to the corresponding alcohol and the carbonyl compound.
In particular, none of the proposed processes considers how to connect the DMP and IPM stages in a practicable manner. This is particularly important against the background that byproducts of the DMP stage have a significant influence on the service life of the catalyst used in the IPM stage. In particular, no process has hitherto been described that envisages connecting the two process stages by simplifying the complex working up caused by the azeotropes.
Since up to now no process is known that describes the production of unsaturated ethers and their precursors, the corresponding ketals and acetals, with recycling of all unreacted substances, the object of this invention was accordingly to provide a process for the production of unsaturated ethers in which the high level of byproducts in the ketal pyrolysis can be tolerated.
A further object of this invention was to provide a process that also tolerates byproducts as well as the pyrolysing ketal in the thermolysis stage.
A particular object of this invention is a process that can operate in the DMP extraction without the addition of a foreign substance, for example an aliphatic hydrocarbon that forms a minimum temperature azeotrope with methanol, and that moreover permits a simple isolation of the unsaturated ether from the methanol-containing and optionally acetone-containing product solution of the pyrolysis.
A further object of this invention was to produce the corresponding ketal(DMP) in high selectivity and yield starting from acetone and methanol and to pyrolyse in a connected process step DMP as well as acetone to the desired product IPM, wherein the separation of water, which greatly interferes in the DMP pyrolysis, should not take place by adding an extraneous additional auxiliary substance.
The invention provides an improved process for the production of unsaturated ethers from the corresponding ketal precursors, the production of these ketal precursors by ketalization of the corresponding carbonyl compound and an aliphatic alcohol, and the advantageous recovery of the product stream of this ketalization by combining it with the product stream of the ketal pyrolysis, followed by extraction of the combined organic product streams with an aqueous sodium hydroxide solution in order to recover unreacted carbonyl compound and/or alcohol.
In particular, this invention is a process for the production of unsaturated ethers of the formula (1) 
by pyrolysis of acetals or ketals of the general formula (2) 
wherein
R1=H or alkyl with 1-8 C atoms; R2=H, CH3xe2x80x94, C2H5xe2x80x94, or Clxe2x80x94; R3=alkyl with 1-8 C atoms; R4=H, CH3xe2x80x94, C2H5xe2x80x94, C3H7xe2x80x94, and R, and R4 may be joined to form a 5-, 6-, or 7-membered ring, characterized in that
a.) the corresponding acetal or ketal of the formula (2) formed from the corresponding alcohol of the general formula (3)
R3xe2x80x94OHxe2x80x83xe2x80x83(3)
xe2x80x83wherein R3 denotes space-holders for the substituents R3 specified above, and from an aldehyde or ketal of the general formula (4) 
xe2x80x83wherein R1, R2 and R3 denote space-holders for the substituents R1, R2 and R3 mentioned above, is ketalized in the presence of an acid catalyst and
b.) the product stream resulting from the ketalization, which consists of a mixture of the components of the formulae (2), (3), (4) and water, is combined with the product stream of the ketal (or acetal) pyrolysis, which consists of ketal (or acetal), the desired unsaturated ether of the formula (1) as product, and the alcohol of the formula (3), and
c.) the combined organic product streams are extracted with an aqueous alkaline solution and the organic product stream of this extraction, which consists of a mixture of the corresponding ketal or acetal, the desired unsaturated ether as product, and residual amounts of the aldehyde or ketone of the formula (4) and water, and
d.) the desired end product is isolated from this largely alcohol-free organic heteroazeotrope product stream by means of distillation, and
e.) the distillation residue, which contains the acetal of the formula (2) and the corresponding carbonyl compound of the formula (4), is pyrolyzed in a high boiling point organic carboxylic acid at 80xc2x0 C.-300xc2x0 C. and the product stream formed thereby, which in addition to the desired ether also contains unreacted ketal and/or acetal and the corresponding alcohol, is fed to the extraction described under b.) and combined with the organic product stream from the ketalization.
Examples of unsaturated ethers of the formula (1) are isopropenyl methyl ether, isopropenyl ethyl ether, isopropenyl propyl ether, isopropenyl isopropyl ether, isopropenyl propyl ether, isopropenyl butyl ether, ethenyl methyl ether, ethenyl ethyl ether, ethenyl propyl ether, ethenyl isopropyl ether, propenyl methyl ether, propenyl ethyl ether, propenyl propyl ether, propenyl isopropyl ether, propenyl butyl ether, wherein isopropenyl methyl ether is particularly preferred.
Examples of acetals or ketals of the formula (2) are dimethoxypropane, acetaldehyde-dimethylacetal, acetalde-hyde-diethylacetal, acetaldehyde-diisopropylacetal, acetaldehyde-dipropylacetal, propionaldehydedimethylace-tal, propionaldehyde-dipropylacetal, propionaldehyde-diisopropylacetal, butyraldehyde-dimethylacetal, butyraldehyde-diethylacetal, butyraldehyde-dipropylacetal, butyraldehyde-dibutylacetal, and corresponding acetals and ketals of aldehydes and ketones mentioned hereinafter of the general formula (4), dimethoxypropane being preferred.
Examples of aliphatic alcohols of the formula (3) are methanol, ethanol, propanol, isopropanol, butanol, pentanol, isopentanol, hexanol, heptanol and octanol, methanol and ethanol being particularly preferred.
Examples of aldehydes and ketals of the formula (4) are acetone, acetaldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde, valeraldehyde, isovaleraldehyde, hexanal, ethylhexanal, methyl isopropyl ketone, methyl ethyl ketone, diethyl ketone, ethyl isopropyl ketone, methyl isobutyl ketone, acetone being preferred.